Fly-By-Wire: The Beginnings

The Second World War witnessed the first applications of computer – controlled fly-by-wire flight control systems. With fly-by-wire, primary control surface movements were directed via electrical signals trans­mitted by wires rather than by the use of mechanical linkages. The German Army’s A-4 rocket (the famous V-2 that postwar was the basis for both U. S. and Soviet efforts to move into space) used an electronic
analog computer that modeled the differential equations governing the missile’s flight control laws. The computer-generated electronic signals were transmitted by wire to direct movement of the actuators that drove graphite vanes located in the rocket motor exhaust. The thrust of the rocket engine was thus vectored as required to stabilize the V-2 missile at lower airspeeds until the aerodynamic control surfaces on the fins became effective.[1107] Postwar, a similar analog computer-controlled fly­by-wire thrust vectoring approach was used in the U. S. Army Redstone missile, perhaps not surprisingly, because Redstone was predominantly designed by a team of German engineers headed by Wernher von Braun of V-2 fame. The Redstone would be used to launch the Mercury space capsule that carried Alan Shepard (the first American into space) in 1961.

The German Mistle (Mistletoe) composite aircraft of late World War II was probably the first example of the use of fly-by-wire for flight con­trol in a manned aircraft application. Mistle consisted of a fighter (usu­ally a Focke-Wulf FW 190) mounted on a support structure on a Junkers Ju 88 bomber.[1108] The Ju 88 was equipped with a 3,500-pound warhead and was intended to be flown to the vicinity of its target by the FW 190 pilot, at which time he would separate from the bomber and evade enemy defenses while the Ju 88 flew into its target. Potentiometers at the base of the FW 190 pilot’s control stick generated electrical commands that were transmitted via wire through the support structure to the bomber. These electrical commands activated electric motors that moved the sys­tem of pushrods leading to the Ju 88 control surfaces.[1109]

Another electronic flight control system innovation related to the fly-by-wire concept had its origins in electronic feedback flight control research that began in Germany in the late 1930s and was published by Ernst Heinkel and Eduard Fischel in 1940. Their research was used in the 1944 development of a directional stability augmentation system for the Luftwaffe’s heavily armed and armored Henschel Hs 129 ground
attack aircraft to compensate for an inherent Dutch roll[1110] instability that affected strafing accuracy with its large-caliber, low-rate-of-fire antitank cannon.[1111] This consisted of modifying the rudder portion of the flight control system for dual mode operation. The rudder was split into two sections, with the lower portion directly linked to the pilot’s flight con­trols. The upper section was electromechanically linked to a gyroscopic yaw rate sensor that automatically provided rudder corrections as yawing motions were detected.[1112] This was the first practical aircraft yaw damper. Northrop incorporated electronic stability augmentation devices into its YB-49 flying wing bomber that first flew in late 1947 in an attempt to compensate for serious directional stability problems. After the war, the NACA Ames Aeronautical Laboratory conducted extensive flight research into artificial stability. An NACA-operated Grumman F6F-3 Hellcat was modified to incorporate roll and yaw rate servos that provided stability augmentation, with flight tests beginning in 1948. In the following years, a number of other aircraft were modified by the NACA at Ames for vari­able stability research, including several variants of the North American F-86.[1113] By the 1950s, most high-performance swept wing jet-powered aircraft were designed with electronic stability augmentation devices.